Four football helmets have earned five stars in the 2013 Virginia Tech Helmet Ratings,TM a test of helmets in a laboratory environment intended to measure their effectiveness in reducing the forces that cause concussion, but critics, especially the National Operating Committee on Standards for Athletic Equipment (NOCSAE), say the results, while helpful, have limitations.
Riddell's 360 continues to be the top rated helmet. Along with its Revolution Speed, the manufacturer holds down two of the four slots in the five star category, Virginia Tech's highest rating awarded to helmets it considers the "best available."  The Rawlings Quantum Plus continues as a 5-star helmet, with Xenith breaking into the elite helmet group with the X2, which jumped from 3 stars.
Two other Rawlings helmets, the Impulse and Quantum, obtained 4-star ratings, joining a group with a Schutt helmet (Ion 4D), two more Riddell models (Revolution and Revolution IQ) and the Xenith X1 classified by Virginia Tech as "very good." A new helmet manufacturer, Simpson-Gnassi, with a background in designing race car helmets, made an impressive debut at the top of the 4-star list with its SG adult helmet.
Schutt held all three spots in the 3-star (good) category with its Air XP, DNA-Pro+ (which inexplicably dropped from 4-stars to 3; see NOCSAE's "concern" about this change below), and the Air XP Ultralite.
Virginia Tech's current evaluation process (which it says will change beginning fall 2014) involves performing 120 impact tests on each helmet model at multiple locations and impact energies utilizing the STAR (Summation of Tests for the Analysis of Risk) Evaluation System, which it developed based on data collected from over 1.8-million head impacts experienced by football players over an eight-year period.
According to Virginia Tech, the STAR value is an estimate of the number of concussions that one collegiate player might experience over the course of a season wearing a specific helmet. The lower the STAR value, the better the helmet is believed by Virginia Tech to be in reducing the risk of concussion.
"We recommend any of the four or five star helmets for players. The specific helmet a player chooses will be dependent on other factors such as a fit and comfort," said project director Stefan Duma, the Harry C. Wyatt Professor of Engineering and department head of the Virginia Tech -- Wake Forest School of Biomedical Engineering and Sciences.
While the Virginia Tech ratings provide parents with valuable biomechanical data intended to help them make educated decisions about which helmet to purchase, parents and athletes need to be aware that the ratings come with a number of significant limitations:
Ratings currently based only on linear acceleration: Because the 2013 STAR values are based solely on linear (i.e. straight-line) acceleration and do not reflect rotational (i.e. twisting) acceleration, Virginia Tech's rating methodology has been heavily criticized by the National Operating Committee on Standards for Athletic Equipment (NOCSAE). In a July 2013 statement, NOCSAE noted that, "A concussion in football is a very complex event involving different and changing forces, linear and rotational accelerations, helmet fit, player position, impact duration, player concussion history and overall health, and potentially even genetics."
The NOCSAE statement goes on to state that "the Virginia Tech Helmet Ratings system approaches the very broad and complex issue of concussion protection from a narrow vantage point of linear accelerations only and does not address other biomechanical variables such as rotational accelerations, particularly where rotational accelerations precede the linear acceleration in a hit. The consensus of scientific experts  agree that rotational accelerations are involved in many, if not almost all, concussive events, although no correlated injury threshold for rotational accelerations has been found."
Responding to the NOCSAE criticism, Virginia Tech initially said that its ratings, beginning in 2014, would take into account rotational (e.g. twisting) acceleration as well, based on a paper published in 2013. New helmets will be added to the STAR ratings in May, and STAR ratings for hockey helmets will be published in fall 2014, which will be the first to incorporate the new methodology including rotational acceleration, according to Duma. STAR ratings using the new methodology for football helmets will be released in 2015, he says.
Values should be used for comparative purposes only: The STAR value is based on injury probability and should not be interpreted as a literal number of injuries for any one athlete.
Wearing a highly rated helmet will not prevent a player from sustaining a concussion. The Virginia Tech researchers concede that any player can sustain a head injury even with the very best protection, that a "specific person's risk of concussion may vary as a result of a number of factors" having nothing to do with the helmets, including genetic differences (some athletes may be genetically predisposed to concussion), age, health history (e.g. history of migraines, depression or other mental health disorders, attention deficit hyperactivity disorder, learning disabilities, sleep disorders, and/or previous history of concussion), impact factors (e.g. neck muscle strength/weight), style of play, etc.